DESCRIPTION: Activation of blood coagulation and deposition of insoluble fibrin invariably contribute to the pathogenesis of vascular injury and the athero-thrombotic disease. Recent studies from the investigator's laboratory have demonstrated that through the interaction with specialized membrane receptors, fibrinogen enhances the adhesion of leukocytes to vascular endothelium by acting as a molecular bridge between the two cell types. Mimicking the earliest atherosclerotic lesion of intraintimal leukocyte infiltration, this adhesion step is followed by monocyte and neutrophil migration across the endothelial cell monolayer. Therefore, a direct role of leukocyte-dependent coagulation in the pathogenesis of vascular injury is hypothesized, and will constitute the focus of the present application. In the first Specific Aim, the ability of plasma adhesive proteins and coagulation factors to support leukocyte-endothelium bridging will be investigated under static or physiologic flow conditions, with respect to cell activation requirements and regulatory pathways of receptor-ligand trafficking. The receptor and ligand structural recognition sites implicated in intercellular bridging will be identified in the second Specific Aim by synthetic peptidyl mimicry, receptor-ligand chemical cross-linking, epitope-mapping of functionally blocking monoclonal antibodies, and rationally targeted site-directed receptor mutants. In the third Specific Aim, the mechanism of leukocyte locomotion across the endothelial cell monolayer will be dissected in its potential requirements of inflammatory/chemotactic cytokines, modulation of activation-dependent genes, and local proteolytic activation of coagulation. The long term objective of these studies is to identify highly selective antagonists of leukocyte adhesion reactions that will reduce vascular damage without affecting cellular hemostatic functions.